The present invention relates to differential and operational amplifiers in general, and more specifically to those amplifiers capable of recovering quickly and without thermally generated distortion from an overdrive condition.
Differential and operational amplifiers typically have a normal dynamic operating range in which a predetermined transfer function of the amplifier is substantially linear. For input voltage swings beyond this range, the amplifier is "overdriven" into a nonlinear region outside of the normal operating range in which active circuit elements can be saturated or cut off. In addition, the overdrive condition can produce a pattern of power dissipation and thermal heating in the active circuit elements that is different from that of the normal linear region.
Consequently, recovery from the nonlinear overdrive condition to the normal operating condition can be delayed by the time needed for saturated active circuit elements to return to a normal operating mode. The recovery can also be delayed, and over a much greater period of time, by the time necessary to restore normal thermal equilibrium and remove thermally induced errors.
Prior art amplifiers capable of recovering from an overdrive condition have included complicated clamping circuits or Darlington feedback transistors that increase the parts count of the circuit.
For example, U.S. Pat. No. 4,691,174 ('174) to Nelson et al entitled "Fast Recovery Amplifier" shows a differential amplifier including identical amplifier halves. In FIG. 5 of the '174 patent, each amplifier has a first feedback path operable in a normal mode including two sets of Darlington transistors (Q5, Q7; Q6, Q8), two diodes (D1, D3; D2, D4), two resistors (R1, R3; R2, R4), and a current source (25). Each amplifier further includes a second feedback path operable in an overdrive condition including a Darlington transistor (Q7; Q8), a diode (D3; D4), two resistors (R1, R3; R2, R4), and a current source (25).
In another example, U.S. Pat. No. 4,714,896 ('896) to Addis entitled "Precision Differential Amplifier having Fast Overdrive Recovery" shows a differential amplifier including identical amplifier halves. In FIG. 2 of the '896 patent, each amplifier has a first feedback path operable in a normal mode that includes a Darlington transistor (Q3 and Q4) and two diodes (D1 and D2). In FIG. 3 of the '896 patent, each amplifier further includes a second feedback path operable in an overdrive condition that includes the first transistor in the Darlington pair (Q3), two different diodes (D3 and D4), and biasing means for diodes D3 and D4 including two current sources (20, 22; 30, 32).
What is desired is an amplifier circuit capable of recovering quickly and without thermally generated distortion from an overdrive condition having a minimum parts count that is less than the parts count of prior art amplifiers.